Download - **Microbial Growth**
**Microbial Growth**
• Growth= an increase in the number of cells, not an increase in size
• Generation=growth by binary fission• Generation time=time it takes for a cell to
divide and the population to double
Bacteria divide by binary fission
Generation times vary for each organism
• E. coli
• Mycobacterium tuberculosis
**Bacterial growth curve**
*Primary and Secondary metabolites
I will ask you about secondary metabolites – know what they are, when they are produced and why.
**Requirements for bacterial growth**
• Environmental factors that influence– Temperature, pH, osmotic pressure, oxygen
• Nutritional factors– Carbon, nitrogen, sulfur, and phosphorous– Trace elements
*Optimum Growth Temperatures
• Psychrophiles-optimum -5 to 15oC• Psychrotrophs- range from 20-30oC• Mesophiles- range from 25-45oC• Thermophiles-range from 45-70oC
Growth rates in different temperatures
pH influences the growth
• Bacteria-prefer a pH range of 6.5-7.5
• Molds prefer a pH range of 5.0-6.0
Osmotic environment influences growth
**Oxygen preferences of organisms**
• Obligate aerobes require oxygen for growth• Facultative anaerobes: can grow in aerobic
or anaerobic environments• Obligate anaerobes: unable to grow in the
presence of oxygen
**Why can some organisms grow in the presence of oxygen?**
• Because they have the ability to neutralize toxic oxygen molecules (superoxide, oxygen radicals)
• Toxic forms of oxygen need to be neutralized by enzymes– Superoxide dismutase– Catalase
*Oxygen Preferences of Bacteria
Candle jars increase CO2 levels for growing capnophiles
How does a candle jar create a CO2-enriched/O2-depleted environment? Is it an anaerobic environment?
Anaerobic jars eliminate the oxygen for anaerobes to grow.
How does the hydrogen gas generator create an anaerobic environment?
Chemical requirements: Carbon
• What are possible sources of carbon?
• Bacteria are classified based on the source of carbon as either heterotrophs or autotrophs
Classification based on Energy and Carbon sources
Chemical Requirements: Nitrogen
• Why do bacteria need nitrogen?
Chemical requirements: Sulfur and Phosphorous
• Why do bacteria need sulfur?
• Why do bacteria need phosphorous?
Chemical requirements: trace elements
• Small amounts of minerals
• Usually function as cofactors
*Culture Media
• Chemically defined– GSA
A chemically defined medium is one where we know EXACTLY what the ingredients are. An undefined medium is one where we don’t know the exact chemical makeup (for instance, it may contain “protein digest”
Chemically defined media
Culture Media
• Complex– Nutrient Agar– TSA– BHI
*Culture Media
• Selective • Differential• Selective and
Differential
*Ways to measure bacterial growth
• Direct Count• Plate Count/Viable Cell Count• Measure Turbidity
Direct Counts
Plate counts require dilutions to obtain colonies
Plate Counts: pour plate
Plate Count-spread plate
*Turbidity gives a rough estimate